The present invention relates to a recording/reproducing system for a magnetic recording medium.
In the field of floppy disk apparatuses, a preceding-erase type magnetic head has recently been devised, which is suitable for use with a high-density magnetic recording medium (disk). In the preceding-erase type magnetic head, an erase head is located in front of a read/write head in the rotational direction of the medium. Before new data are recorded in the medium by the read/write head, previously recorded data are first erased by the erase head. When this type of magnetic head is employed, the erase head should preferably be turned on at a predetermined time before the recording of data on the medium by the read/write head begins, and turned off at a predetermined time before the recording of data end. However, to be able to perform such a control procedure requires a controller, for turning the heads on and off, which is complicated in its construction. The conventional controller is in capable of performing such a control procedure, and for this reason, the erase head and the read/write head are turned on or off simultaneously.
Normally, a track of a magnetic recording medium has a format shown in FIG. 1A, i.e. in which one sector comprises an ID field and a data field.
The ID field comprises a synchronization area SYNC, address mark area AM, identification area ID, and cyclic redundancy check area CRC. Information representative of the starting position and address of a sector are recorded in the ID field.
The data field records data, and comprises synchronization area SYNC, address mark area AM, data area DATA, and cyclic redundancy check area CRC.
Synchronization areas SYNC store synchronization data for a PLL circuit;
address mark AM areas store address mark data indicative of the starting position of the ID field or the data field; PA1 ID area ID stores data such as the cylinder number, the side number, the sector number, and the length of the data field; PA1 data areas DATA store data read out or written in by a user; and PA1 cyclic redundancy check areas CRC store data for checking whether or not the data read out from the ID field or the data field contains an error. PA1 write means for turning said read/write head and said erase head on substantially simultaneously and for supplying a specified signal to said read/write head, thereby to erasing data recorded on said magnetic recording medium, and to write data on said magnetic recording medium; PA1 digital signal-outputting means for outputting a digital signal corresponding to the data read out by said read/write head from said magnetic recording medium; PA1 PLL (Phase Locked Loop) circuit means for receiving the digital signal, and for oscillating in synchronism with the digital signal, thereby to output data pulses and clock pulses; PA1 readout means for receiving the data pulses and the clock pulses and reading out data form said magnetic recording medium in accordance with the data pulses and the clock pulses; PA1 external synchronization detecting means for detecting synchronization data formed of predetermined pattern data, in accordance with the digital signal, and for enabling said PLL circuit means to oscillate in synchronism with the digital signal after the synchronization data has been detected; PA1 internal synchronization detection means for detecting the synchronization data in accordance with the data pulses and the clock pulses output from said PLL circuit means; and PA1 preventing means, for preventing said PLL circuit means from outputting the data pulses and the clock pulses when the digital signal is reproducing from overwritten areas, existing on the tracks of said magnetic recording medium, which have not been erased and in which new data has been written over the previously written data.
Gaps 1, 2, 3, and 4 are areas for absorbing a fluctuation of rotation and an error in a mechanical alignment.
FIG. 1D shows a configuration of a preceding-erase type magnetic head. The magnetic head has an erase head, a read/write head and a separator. The separator magnetically separates the erase head from the read/write head. The erase head is located in front of the read/write head with respect to the rotational direction Z of a recording medium.
Data read out from the recording medium by the magnetic head are fed to a data separator which includes a PLL circuit and an external synchronization detecting circuit.
The external synchronization detector detects the synchronization data in the read data, and upon detection of the synchronization data, the PLL circuit begins oscillating in synchronism with the read data and outputs data pulses and clock pulses.
When the PLL circuit operates in synchronism with the read data, an internal synchronization detector detects the synchronization data on the bases of the data pulses and the clock pulses, and a floppy disk controller checks whether or not the following data are address mark data.
When the address mark data are detected, the floppy disk controller reads out the data stored in the ID area ID or in data area DATA, on the basis of the data pulses and clock pulses. When data other than the address mark data are detected, the floppy disk controller stops operating for a predetermined time period, and then repeats the above-mentioned operation.
FIGS. 1B and 1C show the positions where the erase head and read/write head of the preceding-erase type magnetic head are turned on and off during data are written in the medium. As is shown in FIGS. 1A to 1C, over-written areas are formed in Gap 2, where data are not erased by the erase head and new data are overlapped by recorded on the previously recorded data.
In such a system, when data identical to the synchronization data are recorded in the over-written areas, a problem arise in that an address detection error of the floppy disk controller may occur at the boundary between the over-written areas and the data fields. In other words, when data identical to the synchronization data are recorded in the over-written areas, the floppy disk controller detects the synchronization data at the over-written area, and waits for an address mark data. When the floppy disk controller read out the confused data, it examines whether or not the read-out data is an address mark. When the floppy disk controller judges that the read-out data is not the address mark data, it stops functioning for a predetermined time period.
If the floppy disk controller performs the above operation, it cannot read out the data of the data field which follows the boundary.